Atlas based segmentation in prone breast cancer radiation therapy

The evidence supporting atlas based contour generation is growing and includes breast, prostate, central nervous system, gastrointestinal, gynecologic, and head and neck cancer patient populations. We sought to investigate atlas based segmentation (ABS) in patients with early stage breast cancer status post breast conserving surgery treated with adjuvant radiation therapy in the prone position. An initial atlas library was generated by uploading 20 previously treated patients. Subsequently, a group of 20 consecutive patients underwent treatment planning. Heart, right lung, left lung, total lungs, and breast clinical target volume (CTV) targets were manually contoured per our standard workflow. ABS was then incorporated into our radiation planning workflow and differences in contouring time were recorded, including time needed for ABS volume refinement. ABS generated volumes were compared subjectively in the unedited and edited stages by an independent radiation oncologist to reduce bias by incorporating an interobserver quality analysis. Various objective measurements were used to compare target volume quality including mean distance to agreement (MDA), Dice Coefficient (DC), logit transformation of DC (logit(DC)). The contouring physician edited 88.75% of organ at risk (OAR) volumes on average per patient, whereas the independent reviewing physician recommended revision of 27.5% OAR on average volumes per patient. CTV editing was performed in 20/20 (100%) of cases by the contouring physician, whereas CTV revision was recommended by the independent reviewing physician in 4/20 (20%) of cases. Our atlas performed well with DC values of >0.909 and logit(DC) of >2.344 across heart, lung, and breast volumes when compared to manually generated volumes. All objective measurements demonstrated improvement with physician refinement of ABS generated volumes. The largest absolute improvement was seen in the heart and breast CTV targets. There was 100% acceptance of the edited ABS generated volumes by the independent reviewing physician. The average time saved using ABS was 6.27 minutes (56.92%) per patient. This study confirms ABS offers improvements in efficiency without sacrificing contour quality in the early stage breast cancer patient population and demonstrates the functionality of ABS with prone patient positioning.